Solar cells have been manufactured using a light absorption layer formed at high cost and silicon (Si) as a semiconductor material since an early stage of development. To more economically manufacture commercially viable solar cells, structures of thin film solar cells, using an inexpensive light absorbing material such as copper indium gallium sulfo (di) selenide (CIGS) or Cu(In, Ga)(S, Se)2, have been developed. Such CIGS-based solar cells typically include a rear electrode layer, an n-type junction part, and a p-type light absorption layer. Solar cells including such CIGS layers have a power conversion efficiency of greater than 19%. However, in spite of potential for CIGS-based thin film solar cells, costs and insufficient supply of In are main obstacles to widespread commercial application of thin film solar cells using CIGS-based light absorption layers. Thus, there is an urgent need to develop solar cells using In-free or low-cost universal elements.
Accordingly, as an alternative to the CIGS-based light absorption layer, CZTS(Cu2ZnSn(S,Se)4)-based solar cells including copper (Cu), zinc (Zn), tin (Sn), sulfur (S), or selenium (Se), which are extremely cheap elements, have recently received attention. CZTS has a direct band gap of about 1.0 eV to about 1.5 eV and an absorption coefficient of 104 cm−1 or more, reserves thereof are relatively high, and CZTS uses Sn and Zn, which are inexpensive.
In 1996, CZTS hetero junction PV batteries were reported for the first time, but CZTS-based solar cells have less advanced less than CIGS-based solar cells and photoelectric efficiency of CZTS-based solar cells is 10% or less, much lower than that of CIGS-based solar cells. Thin films of CZTS are prepared by sputtering, hybrid sputtering, pulsed laser deposition, spray pyrolysis, electro-deposition/thermal sulfurization, e-beam processing, Cu/Zn/Sn/thermal sulfurization, and a sol-gel method.
Meanwhile, PCT/US2010/035792 discloses formation of a thin film through heat treatment of ink including CZTS/Se nanoparticles on a base. Generally, when a CZTS thin film is formed with CZTS/Se nanoparticles, it is difficult to enlarge crystal size at a forming process of a thin film due to previously formed small crystals. As such, when each grain is small, interfaces are extended and thereby electron loss occurs at interfaces, and, accordingly, efficiency is deteriorated.
Accordingly, nanoparticles used in a thin film must include Cu, Zn and Sn, and must not be a CZTS crystal type. However, metal nanoparticles constituted of a single metal element may be easily oxidized and, at a subsequent process, an oxygen removal process using a large amount of Se and high temperature is required. In addition, when a chalcogenide including each metal is synthesized respectively and mixed, a non-uniform metal composition ratio may raise a problem. Therefore, there is a high need to develop a technology for thin film solar cells including highly efficient light absorption layers that are stable against oxidation and drawbacks of which are minimized due to a homogenous composition.